Certain applications require extremely clean fluids, including gases, to be transported from a source to a load. Transportation necessarily involves valves which can introduce dirt into the fluid or gas. The present invention prevents intrusion of dirt into the fluid or gas by sealing the valve stem with a metal bellows. A filter is employed which communicates between the exterior and bore of the cover thus equalizing the differential pressure across the bellows. This enables operation of the valve at extremely high pressures.
In valves not having bellows, the main mechanism for the introduction of dirt into the fluid flowing through the valve is the packing which seals the stem of the valve. Valve packing can be made from many different materials. All packing materials tend to disintegrate and generate small particles which pass along the stem and into the transported fluid. The disintegration of the packing occurs when the valve stem moves against the packing as the valve is opened or closed. The small particles can have a disastrous influence on certain processes such as the manufacture of microcomputer chips.
Various valve stem seals employing bellows are known. U.S. Pat. Nos. 4,687,017 and 4,634,099 to Danko et al., U.S. Pat. No. 4,240,610 to Trimble, U.S. Pat. No. 3,097,662 to Peters, and, U.S. Pat. No. 4,688,601 to Astill disclose metallic bellows type valve stem seals. Other valve stem seals employ fluid filled buffer chambers and bellows to effect seals. U.S. Pat. No. 2,880,620 to Bredtchneider discloses the insertion of an incompressible fluid in a bonnet chamber adjacent bellows. Similarly, U.S. Pat. No. 3,826,465 to Whittaker et al. discloses the insertion of an incompressible fluid into two buffer fluid chambers adjacent bellows.
The present invention satisfies the need for a high pressure bellows having approximately no differential pressure across the interior and exterior of the bellows. Approximately zero pressure differential across the interior and exterior of the bellows is accomplished by the filter in the cover and communicating passageways. The equalization of pressure across the wall of the bellows permits operation of the valve at pressures of 30,000 pounds per square inch gage.